This invention relates generally to metal alloys and composite materials having hard coverings thereon. More particularly, the invention relates to articles of cemented carbide, sintered hard alloys, and the like (hereinafter referred to as "super-hard alloys") for cutting purposes and for abrasion or wear resistant mechanical parts, which articles have thin coating layers with excellent abrasion resistance. The invention relates also to a process for producing coated super-hard alloy articles of the above described character.
Heretofore, the practice of covering the working or functional surfaces of super-hard alloy articles with a single layer or multiple layers of various carbides, nitrides, and carbonitrides of group 4a, 5a, and 6a metals of the periodic table, such as titanium carbide and titanium nitride, and/or oxides such as aluminum oxide and zirconium oxide for the purpose of improving the abrasion resistance of these articles has been known. Such coated super-hard alloy articles have been widely reduced to practice as throw-away tips for cutting purposes. Titanium carbide is most widely used as a coating material and, in comparison with the super-hard alloy, itself, has superior properties for the function of a cutting tool such as resistance to oxidation, lubrication property, affinity for iron, and hardness.
However, a super-hard alloy coated with titanium carbide exhibits, as a cutting tool, an excellent performance in resisting flank abrasion, but, on the other hand, the abrasion resistance at the rake face is deficient. This gives rise to a shortening of the serviceable life of the tool.
As a measure for overcoming this problem, the coating of the surfaces of the alloy with aluminum oxide would appear to be suitable. However, coating a super-hard alloy or a covering layer such as titanium carbide with aluminum oxide is not practical since the affinity of the two materials for each other is weak as described hereinafter, and a strong bond cannot be achieved.
As a result of our studies relating to this problem, we have succeeded in solving this problem and have obtained coated super hard alloy articles of excellent resistance to rake face wear by causing a titanium oxycarbide to form on the surface of a super-hard alloy, or a super-hard alloy coated with titanium carbide, by a specific process described hereinafter and forming an aluminum oxide layer on the titanium oxycarbide. We have discovered that by this method, coated super-hard alloy articles of excellent rake face wear resistance can be produced.
Regarding the properties of and research on applications of titanium oxycarbides, the following particulars are known.
1. TiC and TiO have the same crystalline structure (cubic NaCl type) and exist in the form of solid solutions of all proportions as titanium oxycarbide (TiC.sub.x O.sub.y). It is known that the melting point and the lattice constant vary in a continuous manner with the proportions of the C and O. Cf. Reference (1). identified at the end of this specification; PA0 2. As the proportion of the O increases, the free energy of formation of the TiC.sub.x O.sub.y decreases, and the TiC.sub.x O.sub.y becomes a chemically stable compound. According to Carson et al, the lower the free energy of formation is, the greater is the wear resistance of the carbide at high cutting speeds in the case where it is used as a coating on a cutting tool. Cf. Reference (2) PA0 3. The application of TiC.sub.x O.sub.y to cutting tools has already been tried. For example, Carson et al have reported their forming samples of TiC.sub.x O.sub.y by hot pressing mixtures of TiC and TiO powder in various proportions, applying these as coatings onto tool surfaces by RF sputtering, subjecting the tools thus coated to cutting test, and finding that the resulting wear resistance of these tools is approximately three times that of the super-hard tool alloys prior to coating. Cf. Reference (3).
However, sputtering is not adaptable to mass production, produces a weak bond between the coating layer and the super-hard alloy substrate, and can form a coating in only one direction, thus being accompanied by several problems in actual practice.
Another method of forming TiC.sub.x O.sub.y by covering a super-hard alloy of large TiC content with TiO thereby to cause the TiO to diffuse at an elevated temperature is known, but this method is not satisfactory since it forms complex compounds of other metals contained in the super-hard alloy. Cf. Reference (3). Furthermore, in the case where TiC is applied as a coating and is then oxidized, although a portion thereof becomes TiC.sub.x O.sub.y, TiO.sub.2 is readily formed, and the wear resistance of the resulting article is lower than that in the case of TiC.
Thus, while the expedient, per se, of coating super-hard alloys with TiC.sub.x O.sub.y is known, none of the above described known methods have been industrially feasible and have not been developed to practical use.